1. Field of the Invention
The present invention relates generally to methods for operating a hybrid vehicle and, more specifically, to methods for maximizing fuel efficiency while minimizing disruptions in drivability.
2. Description of the Related Art
Most vehicles employ conventional powertrain systems, which rely upon internal combustion engines (ICEs) as the sole source of power. Such systems are powered by fuel energy and, on average, achieve a thermal efficiency of 10–15%, while the remaining 85–90% of the fuel energy is wasted as heat. These heat losses are, in large part, due to the fact that ICEs can only produce energy and not reclaim it. Because conventional powertrain systems cannot recapture the vehicle's kinetic energy, frictional braking is used to slow or stop a vehicle, and in doing so, the vehicle's kinetic energy is converted to lost heat.
Hybrid powertrain systems can mitigate the foregoing efficiency losses. Although (like conventional powertrain systems) hybrid powertrain systems have an ICE powered by fuel energy, they (unlike conventional powertrain systems) also have a “secondary” powertrain system comprised of a two-way energy path capable of capturing the vehicle's kinetic energy during the braking process. Examples of secondary power sources capable of having a two-way energy path include, but are not limited to, electric and/or hydraulic motors. The energy used to power a hybrid's secondary power source (“secondary energy”) depends on the type of secondary power source selected for use, and may consist of, for example, electric energy or hydraulic pressure. This energy is stored in an energy storage device, which receives and stores the vehicle's converted kinetic energy and allows for its reuse, when needed, to power the secondary power source.
When the ICE and the secondary power source of a hybrid vehicle each independently transmits power to the vehicle's wheels, the vehicle is commonly referred to as a parallel hybrid vehicle, and the wheels of the vehicle may be driven solely by the primary power source, solely by the secondary power source, or simultaneously by both. When the driver of such a vehicle makes a demand for power, the determination of which power source is used, and how it is used, greatly influences the vehicle's fuel economy. It also affects the “feel” the vehicle transmits to the driver (“drivability”). For example, when the vehicle is operated according to methods designed to maximize fuel efficiency, the secondary power source may cycle on and off too frequently, causing the drivability of the vehicle to suffer, resulting in a disruptive feel that may be commercially unacceptable to consumers. Conversely, when the vehicle is operated in a manner designed to maximize drivability, the secondary power source may not be used often enough, thereby resulting in a lower fuel economy than may be otherwise possible.
As a result, there is a need for a new and improved method of operating a parallel hybrid vehicle in a manner that strikes a balance between maximizing the vehicle's fuel efficiency while still providing the driver with an acceptable driving experience.